The removal of dust as a simple household task from domestic dwellings has been and is still carried out with dust cloths. In more sophisticated situations, chatelaines and housekeepers have employed maids, butlers and valets equipped with white gloves or mittens to remove dust, polish silverware and furniture. Previously, there has been no necessity or interest in analyzing the collected dust particles. Rather, the sole intent was to collect and throw out dust and dirt particles.
The necessity to collect dust and particles for analysis is a more recent requirement. With the advent of terrorism where explosives can be concealed to create undetectable bombs and also with drug smugglers concealing their drug shipments, the need for trace/forensic detection has become necessary. While early explosives detectors relied on the collection of atmospheric explosives vapours, modern high power explosives are plastic with extremely low vapour pressures thereby presenting vapour detection instruments with extremely difficult detection situations to the point of impossibility. Modern plastic explosives can have vapour pressures 5 or 6 orders of magnitude lower than the traditional volatile explosives such as nitroglycerine (NG), trinitrotoluene (TNT) and ethylene glycol denitrate (EGDN). For example, U.S. Pat. No. 4,909,090 teaches the use of hand operated vapour samplers which heats the surface to assist in dislodging vapours which are trapped on collector surfaces in the probe. However, with the very low vapour pressure of plastic explosives and drugs this method is extremely limited in its usefulness. Drugs like cocaine, heroine and others also have very low vapour pressures and thus are undetectable by existing vapour detection instruments.
Modern detection equipment can detect extremely low levels of explosives, narcotics and the like, in the range of nanograms or picograms. However, this simply raises the problem of obtaining a suitable sample. Accordingly, rather than attempt to collect a vapour sample, an alternative approach of particle collection and analysis was proposed, notably by Barringer, et al, and followed by others, both for forensics purposes as well as for surface geochemical exploration where trace metals and organometals can be useful as pathfinder indicators in mineral exploration activities. Earlier patents have covered these applications, such as Barringer U.S. Pat. Nos. 3,970,428; 4,220,414; 4,192,176; and 5,425,263.
Particle collection techniques include: surface inspection by means of physical particle collection in minute amounts; the use of dust pan-brush arrangements; vacuum suction onto porous or semi-porous substrates, filters, membranes and the like; the use of swabs, swipes, gloves, etc. One such method is described in U.S. Pat. No. 5,476,794 which describes the removal of particles with a glove and the use of an intermediate step, that of vacuum suction off the glove. These techniques have been the subject of earlier patents, which have developed into more sophisticated techniques incorporated into various instrumental devices for direct analysis, including plasma optical emission spectrometers, optical analyzers and mass spectrometers among others.
Gloves, mitts and swipes have been used in various forms for particle collection. Disadvantages exist with these earlier systems of collection. The major disadvantage of most of these earlier techniques is that an intermediate step is required to transfer from such a glove or the like any particles/trace chemicals so collected for presentation to the analytical device. One method is to use a suction device to vacuum the glove or mitt, as in U.S. Pat. Nos. 5,476,794. Not only does this present a loss of time and cause nuisance to the operator, but the secondary stage is inefficient, causing a loss of sample due to incomplete transfer from the glove. Additionally, vacuum suction devices are noisy, cumbersome, and require power to energize the suction motors. Even small vacuum hand samplers have relatively limited battery life. If a positive hit is obtained, then usually the suction device is contaminated and has to be cleaned thoroughly before being used again. Finally, often an even greater problem is created by the suction causing glove/cloth fibers and lint to be released which can either obstruct the collecting substrate, present interfering chemicals or fluff/lint which might compete in the analytical process, as for example, if IMS is used where matrix effects from the hand covering material may well compete too aggressively in the ionization process.
Sampling materials or cloth which cover the fingers are also known for collecting particles from surfaces. These have the advantage of avoiding the intermediate step and use of a suction device. Such sampling cloths or finger covers unfortunately leave the fingers vulnerable to damage from sharp objects or exposed corners, hot objects or surfaces contaminated with toxic chemicals. Finger swabs/hand mitts can also become dislodged during search operations when pressed into tight areas.
Finally the problem of cross-contamination remains, in that if a hand/finger is in direct contact with the swab, or glove/mitt, there is always the possibility that the hand/fingers will become contaminated with the analyte sought and pass this over as false positive alarms or hits onto subsequent samples. Also in some sampling situations, such as Chemical Warfare (CW) agents/liquid droplets, the chemical toxicity of the sample is too dangerous to allow close contact with hands.
As described in U.S. Pat. No. 5,476,794, particles are transferred from the glove to a collection probe, and the complete probe is inserted into the analyzer to vaporize the samples. The major disadvantage with this technique is the complexity of the sampling probe, the ease with which it can be clogged by debris and lint off gloves, and its complexity and hence cost.
An alternative means to collect trace particles is to use small filter discs which are inserted into the suction line of a vacuum cleaner unit to efficiently remove by suction the particles required for analysis. The substrate discs are positioned immediately at the sampling head to prevent particles lodging in the suction line and becoming dislodged at a later time thereby giving false anomalies. The collection substrate must be porous enough to allow air suction while at the same time be sufficiently dense to entrap the particles in the size range of interest. After a collection time adequate to collect a sufficient quantity of dust/material for analysis, the filter disc or substrate is removed and presented for analysis, as for example to an ion mobility spectrometer (IMS). The filter disc is inserted into a thermal desorption device which is rapidly heated to volatilize the collected material. The heating process converts the trace particles to vapours for conventional chemical vapour analysis, such as might be made by an IMS, mass spectrometer or gas chromatography or such other instrument.
The disadvantage of this technique is that it is still possible that the vacuum cleaner can become contaminated. More importantly, it requires manipulating a cumbersome vacuum cleaner to obtain a sample. Bearing in mind that only trace samples may be available, it is highly desirable to be able to quickly and efficiently gather particles from the surfaces of luggage, vehicles etc.